A silicon photomultiplier (SiPM) is a photodetector element including two-dimensionally arranged avalanche photodiodes (hereafter referred to as “APDs”), which operate in a mode called “Geiger mode” when a reverse-bias voltage higher than a breakdown voltage of the APDs is applied thereto. The gain of an APD operating in Geiger mode is very high, 1×105 to 1×106. Therefore, a very weak light emission of a single photon can be measured using the APD.
A resistor having a high resistance value called “quenching resistor” is connected in series to each APD of a SiPM, When a single photon enters the APD to cause a Geiger discharge, the quenching resistor causes a voltage drop to terminate the amplification. As a result, a pulsed output signal can be obtained. Each APD of the SiPM acts in this manner. Accordingly, if the Geiger discharge occurs in a plurality of APDs, an output signal can be obtained, the output signal indicating a charge value or pulse height value obtained by multiplying an output signal of a single APD by the number of APDs in which Geiger discharge occurs. Therefore, the number of APDs in which the Geiger discharge occurs, i.e., the number of photons entering the SiPM, can be determined from such an output signal. This enables the counting of the number of photons.
As described above, if a plurality of photons enters the SiPM, the number of photons can be correctly counted as long as a single photon enters each APD of an APD array, since the Geiger discharge occurs in each APD. However, it takes some time for an APD in which a Geiger discharge occurs to recover to the original reverse-bias potential state. If a photon enters thereto during such a time, a sufficient reverse-bias is not applied to the APD. As a result, the photon is not counted. Therefore, the recovery time is called “dead time.” If a large number of photons reach the APD array during the dead time, there would be a loss in the counting of photons. Accordingly, the output signal shows nonlinear values relative to the number of photons. As a result, the photon counting accuracy is considerably degraded.
The spatial distribution and temporal distribution of the photons entering the SiPM greatly relate to the cause of such a degradation. For example, cases where light rays having the same energy enter the SiPM uniformly and non-uniformly are considered. If the light rays enter non-uniformly, a frequency with which a single APD receives a light ray during a short period increases. Accordingly, the output signal in such a case becomes lower than that in the case where the light rays enter uniformly. However, the APDs of the SiPM are connected in parallel, and there is no information on which APDs are in the Geiger mode at which timing. Therefore, such an output signal, which is an erroneous signal having information that a lower number than the actual number of photons enter, the energy resolution of the SiPM is degraded.
In order to improve the characteristics of SiPMs, the number of APD arrays for receiving photons is increased, or the dead time is shortened in some SiPMs. However, if the number of APD arrays is increased, the area of each APD may be reduced. This would degrade the photon detection efficiency and the gain. The shortening of dead time is in a trade-off with an increase of noise or a decrease of gain. Accordingly, this cannot solve the problem fundamentally.